1. Field of the Invention
This invention relates to the field of orthopedic surgery and, particularly, to a cross link attachable to a pair of spinal rods to stabilize relative movement between the rods during body movements.
2. Description of the Prior Art
The use of spinal rods is conventional for correction of spinal trauma or conditions, such as curvature of the spine. Generally parallel rods are attached to the spine by pedicle screws for support or correction of abnormalities. During normal activities, the spine is subjected to bending and twisting motions which affect the relative position of the spinal rods. In some cases, the rods are cross braced for additional stability. The cross link is attached to each of the spinal rods and provides fixed spacing between the rods. The resultant forces acting on the cross link are primarily in shear along the longitudinal axis of the cross link.
Estes, U.S. Pat. No. 5,693,053, teaches the use of a rigid cross link with an eye on each end. The eyes are oblong to provide some adjustability in length of the cross link. Bolts connected to the spinal rods are passed through the eyes. Nuts are then applied to hold the assembly in place. The shear forces tend to cause the bolts to move within the oblong eyes.
U.S. Pat. No. 5,275,600 to Allard et al teaches a two piece cross link with telescoping members. Each end of the two piece rod has a hook for engaging the spinal rods. Each hook has a downwardly facing opening in which the spinal rod is captured. The opposing sides of the hook resist the shear forces however, there is nothing to prevent the hooks from, “walking,” along the spinal rods in response to unequal movement of each spinal rod. The other end of one rod is telescoped into the second rod. A set screw is threaded through the telescoped portions engaging both members and fixing the length of the cross link. The set screw is threaded through the exterior rod and the interior rod has a hook or flange to prevent separation. The ends of the telescoped rods act directly on the shaft of the set screw in shear which may result in deformation or failure.
Lin et al, U.S. Pat. No. 6,302,882, teach a cross link with two shafts that overlap each other. Each shaft has a double hook on one end for engaging a spinal rod. The double hook enlarges the contact area between the cross link and the spinal rods to resist, “walking,” along the spinal rods. The other ends of the shafts each have a reduced thickness and are overlapped. A clip or sleeve is placed over the overlapped portions. The clip has a smaller diameter than the diameter of the overlapped portions which creates a force fit. A set screw may be used to secure the clip on the overlapped shafts. The shear forces can move the overlapped portions within the clip since there is no transverse fastening in the assembly.
Because of the forces acting along the cross link and the movement of the spinal rods, the connection between the cross link and the spinal rod must be secure to avoid movement of the cross link along the spinal rod. Further, the connection between multi-shaft cross links must resist shear to avoid separation, in the worst case, and to provide stability to the spinal rods.